Method for dispersing optical brighteners in waterborne coating compositions

ABSTRACT

This invention provides a method for rendering a water-based coating (i.e., one which contains essentially no organic solvent) fluorescent. Also provided is a method for measuring the extent of coverage and thickness of a water-based coating by incorporating therein a minor amount of an emulsion of an insoluble compound which absorbs ultraviolet light and fluoresces strongly in the visible region of the spectrum, so that the intensity of the fluorescence serves as an indication of the degree of coverage and thickness of the film. Also provided is a method for enhancing the apparent brightness of a film pigmented with light colored pigments. In the case where white pigments such as titanium dioxide are used, the effect is to increase the apparent whiteness of the film.

This is a divisional application of application Ser. No. 08/544,331,filed Oct. 17, 1995, U.S. Pat. No. 5,650,455.

FIELD OF THE INVENTION

This invention relates to the field of coating compositions. Inparticular, it relates to a method for dispersing otherwise insolubleoptical brightener compounds in aqueous coating compositions.

BACKGROUND OF THE INVENTION

There are many occasions in which it is necessary to apply a coating toa substrate in a thin, very uniform film, and in which it is desired tohave a simple method for measuring the film thickness or, in the case oftransparent films, to render the film visible so that a concise coatingmay be assured. This is particularly important when, for example, a filmof an adhesion promoter is applied to a substrate to which a finalfinish coat is to be applied. In such a case, it is necessary to ensurethat the substrate is coated completely and at the same time, that theadhesion promoting film is neither too thin nor too thick to givesatisfactory appearance and adhesion of the finish coat.

The use of fluorescent compounds, including several of the so-called"optical brighteners" as indicators of the presence of a film, and ofits thickness, is well known to the art. However, none of the prior artmethods are applicable to coatings which are essentially free of organicsolvents, since the coating compositions generally require that thefluorescent compound be soluble in an organic solvent. Unfortunately,some of the most efficient fluorescers such as thebis(benzoxazolyl)stilbenes and their homologs are, for all practicalpurposes, insoluble in water.

U.S. Pat. No. 4,741,860 describes a solvent-borne coating compositionwhich contains a chlorinated poly-olefin, at least 0.1% by weight, basedon the total weight of the composition, of an optical brightener. Thisreference teaches that the use of non-polar solvents is preferred.

EP 303803 describes a method for mixing colorants into a stable aqueoussuspension of crosslinked polymer.

EP 336029 teaches polycyclic fluorescent compounds which absorbultraviolet light in the range of 240 to 400 nanometers, and whosefluorescence is preferably below 400 nm. This reference teaches thatless than 30% of the fluorescence energy should be above 400 nm. Thecompounds described are either relatively low molecular weight monomerichydrocarbons or, preferably, functionalized fluorescent compounds whichare reacted to form a part of the substrate backbone. Fluorescence inthe visible region of the spectrum (i.e. 400 nm and above) is consideredundesirable in the application of this reference. It is further statedthat the fluorescent compounds should be soluble in the coating.

Water-soluble fluorescent compounds are known; e.g., optical brightenersused in detergents for increasing apparatus whiteness. However, thecommon water-soluble optical brighteners have insufficient thermalstability to be useful in coating compositions. Further, suchwater-soluble fluorescent compounds generally contain sodium sulfonateor amino functional groups which may be reactable with other typicalcomponents in coating compositions.

SUMMARY OF THE INVENTION

One object of this invention is to provide a method for rendering awater based coating (i.e., one which contains essentially no organicsolvent) fluorescent. Another object of the invention is to provide amethod for measuring the extent of coverage and thickness of awater-based coating by incorporating therein a minor amount of anemulsion of an insoluble compound which absorbs ultraviolet light andfluoresces strongly in the visible region of the spectrum, so that theintensity of the fluorescence serves as an indication of the degree ofcoverage and thickness of the film. Still another object of theinvention is to provide a method for enhancing the apparent brightnessof a film pigmented with light colored pigments. In the case where whitepigments such as titanium dioxide are used, the effect is to increasethe apparent whiteness of the film. One further object of the inventionis to provide an adhesion promoting coating comprising a chlorinatedpolyolefin or maleated polyolefin, emulsifying agents, and an insolubleoptical brightener, such as Eastman Chemical Company's EASTOBRITE® OB-1,which has been rendered water-dispersible by incorporation into anemulsifiable polyolefin wax.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an aqueous emulsion comprising

(a) a low molecular weight emulsifiable polyolefin wax;

(b) a non-ionic surfactant;

(c) water; and

(d) an essentially water-insoluble optical brightener compound.

In the above emulsion, the low molecular weight emulsifiable polyolefinwax is any wax which is capable of ready emulsion, for example, theoxidized polyolefins and modified polyolefins such as chlorinatedpolyolefins and maleated polyolefins. Such a wax preferably has adensity in the range of about 0.92-0.96, melt viscosities in the rangeof about 50-4000 cp at 125° C., and an acid number in the range of about12-55. Exemplary waxes include an oxidized polyethylene wax having adensity of 0.939, a melt viscosity of 900 cp at 125° C., and an acidnumber of about 15; an oxidized poly-ethylene wax having a density of0.955, a melt viscosity of 250 cp at 125° C., and an acid number ofabout 16; and a maleated polypropylene wax having a density of 0.934, amelt viscosity of 400 cp at 190° C., and an acid number of about 47.

Preferred examples of non-ionic surfactants include nonionic surfactantssuch as alkyl polyglycol ethers such as ethoxylation products of lauryl,oleyl, and stearyl alcohols; and alkyl phenol polyglycol ethers such asethoxylation products of octyl- or nonylphenol, diisopropyl phenol,triisopropyl phenol. A preferred nonionic surfactant is TERGITOL15-S-15, which is an ethoxylated linear alcohol having ahydrophilic-lipophilic balance of 15.4 as determined according toGriffin, W. C. , Office. Dig. Federation Paint Varnish Prod. Blubs, 28,446 (1956).

The fluorescent or optical brightener compounds of this invention arewater-insoluble compounds which absorb ultraviolet light of wavelengthsin the range from about 450 to about 480 nm and fluoresce in the rangeof about 420 to 450 nm. Preferred compounds arebis(benzoxazolyl)stilbenes of the general formula in which R¹ and R² are##STR1## independently hydrogen; halogen; C₁ -C₄ alkyl; and C₁ -C₆alkoxycarbonyl. Especially preferred compounds include compounds of thisformula wherein R¹ and R² are independently hydrogen or methyl.Preferred optical brighteners include Eastman's EASTOBRITE® OB-1, CibaGeigy's UVITEX® MES and UVITEX® OB, Hoechst KS, Hoechst KSN, andLeucopure® EGM from Sandoz.

Although this invention is directed primarily to the use of opticalbrighteners of the bis(benzoxazolyl)-stilbene class, it should beunderstood that in general any optical brightener, defined as a compoundwhich absorbs ultraviolet light and fluoresces in the visible region ofthe spectrum, which is essentially insoluble in water and which can beincorporated into an emulsion with an oxidized polyolefin wax may beused. As used herein, the phrase "essentially insoluble" means acompound which has a solubility in water of less than about 0.3 g/L at24° C. Preferably, the fluorescent compound absorbs ultraviolet light ofwavelength shorter than 400 nm, and emits light at a wavelength aboveabout 420 nm. Compounds which absorb light of wavelength above about 400nm are colored and would affect the color of the applied coating. Sinceboth absorption and emission cover a range of wavelengths, the rangesabove refer to the region over which about 80% of the absorption andemission occur.

The concentration of the fluorescent compound in the final coatingformulation may be varied widely, preferably from about 5 ppm or less toas much as 500 ppm. In general, the lowest concentration which will givethe desired degree of fluorescence should be used, since the inherentcolor of the brightener will become more apparent as the concentrationincreases. For most purposes, the preferred concentration of fluorescentcompound in the coating is between 100 and 450 ppm.

The objects of the invention can be accomplished by incorporating into awater-based coating composition an aqueous emulsion of a low molecularweight oxidized polyolefin wax which contains a water-insoluble opticalbrightener.

An example of a preferred aqueous emulsion containing the opticalbrightener has the following composition:

    ______________________________________                                                               Wt. %,                                                 Material               Range                                                  ______________________________________                                        Low. mol. wt. oxidized 3.0-8.0                                                polyolefin wax                                                                Nonionic Surfactants   2.0-4.0                                                Potassium hydroxide    0.1-0.2                                                Silicone antifoam      0.2-0.4                                                Antimicrobials         0.0-0.2                                                Deionized water        37.0-69.6                                              EASTOBRITE ® OB-1  25.0-50.0                                              ______________________________________                                    

The maximum concentration of the optical brightener is limited by theviscosity of the emulsion, which must be low enough to permit theemulsion to be stirred into the coating system.

The coatings of this invention are useful primarily as undercoatings,especially for coatings which serve as adhesion promoters between twodifferent substrates such as polyolefin moldings used in automobilebodies, and the finish paint. However, that the principle of rendering awater-based coating fluorescent is generally applicable. Such coatingsmight be used for aesthetic reasons to enhance the apparent whiteness ofwhite coatings, or the brightness of pastel paints. The principle ofusing an aqueous dispersion of an optical brightener, as provided bythis invention is not restricted to undercoatings.

Thus, as a further aspect of the present invention, there is provided anadhesion promoting composition comprising

(I) a chlorinated or maleated polyolefin;

(II) an aqueous emulsion comprising

(a) a low molecular weight emulsifiable polyolefin wax;

(b) a non-ionic surfactant;

(c) water; and

(d) an essentially water-insoluble optical brightener compound.

If fluorescence is to be used as an indication of film uniformity andthickness, it is essential that the fluorescent material, if notactually dissolved in the coating, be present in uniform, exceedinglyfinely dispersed particles, and that it not settle or segregate onstorage, or be readily stirred into a dispersion state.

As used herein, the term "chlorinated polyolefin" refers to achlorinated polyolefin having a number average molecular weight in therange of 9,000 to 45,000, a softening point in the range of 75° C. to105° C., and an amount of chlorine in the range of 15 to 35 weightpercent based on the weight of the polyolefin. One embodiment of thechlorinated polyolefin useful in this invention is disclosed in U.S.Pat. No. 3,579,485. Another useful chlorinated polyolefin is thischlorinated polyolefin reacted with a hydroxyl group containing primaryamine to form a chlorinated, imidized polyolefin. These polyolefins areknown and described in U.S. Pat. No. 4,954,573. In a preferredembodiment, the chlorinated polyolefin has a number average molecularweight in the range of 9,000 to 16,000, a softening point in the rangeof 80° C. to 95° C., and an amount of chlorine in the range of 18 to 22percent, based on the weight of the polyolefin. Further examples ofchlorinated polyolefins can be found in U.S. Pat. Nos. 3,579,485;4,070,421; 4,966,947; 4,962,149; 5,198,485; and 4,954,573, incorporatedherein by reference.

As used herein, the term "maleated polyolefin" refers to a polyolefinwhich has been functionalized by reaction with compounds such as maleicanhydride. Examples include maleated polybutylene, maleatedpolypropylene, and maleated polyethylene. Examples ofcommercially-available maleated polyolefins includes POLYBOND®,available from BP Performance Polymers, Inc., MAPO maleated polyolefindispersion, available from Eastman Chemical Company, and from AlliedChemicals, Polyace® 573. Further examples of maleated polyolefins can befound in U.S. Pat. Nos. 4,874,818; 4,338,230; 3,919,176; and 5,281,670,incorporated herein by reference.

The adhesion promoting compositions optionally contain other typicaladditives such as antioxidants, including, for example, hinderedphenols, thioesters, organophosphites, and hindered amines, which mayreadily be dispersed or emulsified in the aqueous emulsion system.Additional additives include coupling agents, antistatic agents,nucleating agents, metal deactivators, lubricants, slip agents,antiblocking agents, uv inhibitors, flame retardants, and the like.

The problem with obtaining stable dispersions is reflected in the priorart, in which only organic solvent-based systems or water-solublefluorescers are used. We have found no reference which suggests thepossibility that such an insoluble, extremely fluorescent compound as4,4' (bisbenzoxazolyl)stilbene (e.g., EASTOBRITE® OB-1) could bedispersed evenly enough in a water-based system to be used as afluorescent indicator of film thickness. Indeed, many attempts toprepare stable dispersions of this brightener by conventional means havebeen unsuccessful. Optical brighteners of this class are normally usedin synthetic fibers or plastics, into which they are incorporated athigh temperature. Their use in coatings at low temperatures has hithertobeen considered impossible.

Thus, as a further aspect of the present invention, there is provided amethod for determining the thickness and coverage of a water-bornecoating composition, which comprises applying a water-borne coatingcomposition having therein, at least about 0.01 weight percent of anaqueous emulsion comprising

(a) a low molecular weight emulsifiable polyolefin wax;

(b) a non-ionic surfactant;

(c) water; and

(d) an essentially water-insoluble optical brightener compound; to asubstrate and measuring the fluorescence of said coating upon exposureto ultraviolet light.

The invention is not limited to waterborne adhesion promoters, but isalso generally applicable to any lightly-pigmented water-borne finish,whether used as an undercoat or as a finish coat, as illustrated inExample 3 below.

Thus, as a further aspect of the present invention, there is provided amethod for improving the apparent brightness of a lightly-pigmentedwaterborne coating composition, which comprises incorporating in saidcomposition, an aqueous emulsion comprising

(a) a low molecular weight emulsifiable polyolefin wax;

(b) a non-ionic surfactant;

(c) water; and

(d) an essentially water-insoluble optical brightener compound.

EXPERIMENTAL SECTION Example 1

This examples illustrates the application of the process of thisinvention in a film prepared from a chlorinated polyolefin adhesionpromoter.

A coating was prepared from an oil in water emulsion of a chlorinatedpolyolefin (Eastman's CPO) which contained 24% of chlorinatedpolyolefin, water, no organic solvent, and less than 3% of ammonia as aneutralizing amine. To each of three 25 g samples of this emulsion wasadded 10 g of water and then none, 2 drops, and 5 drops of a 50%dispersion of Eastman's EASTOBRITE® OB-1 dispersed in water and oxidizedpolyolefin wax as described above.

Each sample was shaken to mix the contents, then drawn down onto "Form70" Laneta paper to give a 1.5 mil wet film. The films were dried at 50°C., then viewed under an ultraviolet lamp. There was no fluorescence inthe control sample; the sample which contained 2 and 5 drops of thefluorescent brightener dispersion showed strong fluorescence; the degreeof fluorescence increased with the amount of brightener dispersionpresent.

It was noted particularly in these experiments that the fluorescence wascompletely uniform; there was no indication of "clumping" of thefluorescent compound in the film which would have indicated an unevendistribution of the fluorescent compound.

Example 2

This example illustrates the visual effect of increased coatingthickness on film appearance of a chlorinated polyolefin film whenviewed under ultraviolet light.

A coating was prepared from the chlorinated polyolefin dispersiondescribed in Example 1, (20 g) and 0.02 g of the EASTOBRITE® OB-1dispersion. The components were agitated sufficiently to mix themthoroughly, then drawn down onto cold rolled steel and dried. Dry filmthicknesses of 0.194, 0.419, 0.653, 0.781, 0.847, and 1.111 mils wereobtained. When these films were viewed under ultraviolet light, asmooth, definite increase in fluorescence was obtained with eachincrease in thickness of the film.

Example 3

This example illustrates the effectiveness of the process of thisinvention in imparting fluorescence to a pigmented and unpigmentedmelamine baking enamel. coatings were prepared from pigmented andunpigmented melamine baking enamels. In each case, 20 g of enamel (7.6 gof solids) and 0.02 g of the EASTOBRITE® OB-1 dispersion were combined,mixed thoroughly, and drawn down onto cold rolled steel. The films werecured by baking and observed under ultraviolet light. Film thicknessesof 0.223, 0.393, 0.550, 0.724, 0.831, and 1.206 mils were obtained. Inboth the pigmented and unpigmented enamels, a smooth increase influorescence with film thickness was observed.

Example 4

This example shows that uniform and usefully fluorescent coatings areeasily prepared which contain between 25 ppm and 500 ppm of EASTOBRITE®OB-1.

Films were prepared as described previously from the chlorinatedpolyolefin dispersion described in Example 1 which contained,respectively, 500, 250, 100, and 25 ppm of EASTOBRITE® OB-1. Theingredients were mixed, then allowed to roll on rollers for 4 hours,then drawn down onto cold rolled B1000 steel panels, using a No. 7 wirerod. The coatings were observed under an ultraviolet light after theyhad air-dried for 72 hours. All of the coatings were fluorescent. It wasjudged that a concentration of 100 ppm of brightener would be adequatefor practical purposes.

Example 5

Dispersions were prepared as described in the table above whichcontained, instead of EASTOBRITE® OB-1, the same amounts of Uvitex® MESand of Leucopure® EGM, respectively. When these were mixed into thechlorinated polyolefin dispersion at concentrations of 500 and 1500 ppm,respectively, both gave smooth, uniformly fluorescent coatings whencoated onto steel plates as described in Example 4.

Example 6

Attempts were made to dispense without the oxidized polyolefin waxdispersing agent used in the previous examples by simply mixingEASTOBRITE® OB-1 or Uvitex® MES into the chlorinated polyolefin coatingformulation. At brightener concentrations of 250 ppm, the brightenersdid not disperse well into the coating formulation, and the resultingcoatings exhibited nonuniform fluorescence.

Example 7

This example illustrates the effectiveness of the EASTOBRITE® OB-1dispersion in a coating formulation prepared from an aqueous dispersionof a maleated polyolefin, rather than the chlorinated polyolefindispersion used in Examples 1-6. The dispersion was prepared as before,using 20 g of Eastman MAPO maleated polyolefin dispersion and 0.002 g.of the 50% dispersion of EASTOBRITE® OB-1 in oxidized polyolefin wax. Auniform dispersion was obtained, which, drawn down onto cold rolledsteel as usual, gave a uniformly fluorescent coating.

We claim:
 1. An aqueous emulsion comprising(a) a low molecular weightemulsifiable polyolefin wax in the amount of 3.0 to 8.0 weight percentbased on the weight of the aqueous emulsion; (b) a non-ionic surfactantin the amount of 2.0 to 4.0 weight percent based on the weight of theaqueous emulsion; (c) water in the amount of 37.0 to 69.6 weight percentbased on the weight of the aqueous emulsion; and (d) an essentiallywater-insoluble optical brightener compound in the amount of 25.0 to50.0 weight percent based on the weight of the aqueous emulsion.
 2. Theemulsion of claim 1, wherein the low molecular weight emulsifiablepolyolefin wax is a wax having a density in the range of about0.92-0.96, melt viscosities in the range of about 50-4000 cp at 125° C.,and an acid number in the range of about 12-55.
 3. The emulsion of claim1, wherein the optical brightener compound is a compound having theformula ##STR2## wherein R¹ and R² are independently hydrogen; halogen;C₁ -C₄ alkyl; or C₁ -C₆ alkoxycarbonyl.
 4. The emulsion of claim 3,wherein R¹ and R² are hydrogen.
 5. The emulsion of claim 3, wherein R¹and R² are methyl.
 6. A method for determining the thickness andcoverage of a waterborne coating composition, which comprises applying awaterborne coating composition having therein, at least about 0.01weight percent of an aqueous emulsion comprising(a) a low molecularweight emulsifiable polyolefin wax in the amount of 3.0 to 8.0 weightpercent based on the weight of the aqueous emulsion; (b) a non-ionicsurfactant in the amount of 2.0 to 4.0 weight percent based on theweight of the aqueous emulsion; (c) water in the amount of 37.0 to 69.6weight percent based on the weight of the aqueous emulsion; and (d) anessentially water-insoluble optical brightener compound in the amount of25.0 to 50.0 weight percent based on the weight of the aqueous emulsion.7. The method of claim 6, wherein the low molecular weight emulsifiablepolyolefin wax is a wax having a density in the range of about0.92-0.96, melt viscosities in the range of about 50-4000 cp at 125° C.,and an acid number in the range of about 12-55.
 8. The method of claim6, wherein the optical brightener compound is a compound having theformula ##STR3## wherein R¹ and R² are independently hydrogen; halogen;C₁ -C₄ alkyl; or C₁ -C₆ alkoxycarbonyl.
 9. The method of claim 6,wherein R¹ and R² are hydrogen.
 10. A method for improving the apparentbrightness of a lightly-pigmented waterborne coating composition, whichcomprises incorporating in said composition, an aqueous emulsioncomprising(a) a low molecular weight emulsifiable polyolefin wax in theamount of 3.0 to 8.0 weight percent based on the weight of the aqueousemulsion; (b) a non-ionic surfactant in the amount of 2.0 to 4.0 weightpercent based on the weight of the aqueous emulsion; (c) water in theamount of 37.0 to 69.6 weight percent based on the weight of the aqueousemulsion; and (d) an essentially water-insoluble optical brightenercompound in the amount of 25.0 to 50.0 weight percent based on theweight of the aqueous emulsion.
 11. The method of claim 10, wherein thelow molecular weight emulsifiable polyolefin wax is a wax having adensity in the range of about 0.92-0.96, melt viscosities in the rangeof about 50-4000 cp at 125° C., and an acid number in the range of about12-55.
 12. The method of claim 10, wherein the optical brightenercompound is a compound having the formula ##STR4## wherein R¹ and R² areindependently hydrogen; halogen; C₁ -C₄ alkyl; or C₁ -C₆ alkoxycarbonyl.13. The method of claim 12, wherein R¹ and R² are hydrogen.
 14. Themethod of claim 12, wherein R¹ and R² are methyl.